In recent years, an electrochemical device, particularly a lithium secondary battery is widely used for a small-sized electronic equipment such as a cellular phone and a laptop computer, an electric vehicle or storage of the electric power. These electronic equipments or a vehicle is likely to be used in a broad temperature range of high temperature in the midsummer, low temperature in the arctic weather, and the like, and thus it is required to improve the electrochemical properties in a broad temperature range with a good balance.
Particularly in order to prevent global warming, it is urgently needed to cut CO2 discharge, and immediate diffusion of a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or a battery electric vehicle (BEV) is demanded, among environment-friendly cars loaded with an electrical storage device including an electrochemical device such as a lithium secondary battery and a capacitor. However, a vehicle has long migration length, and thus is likely used in a region of broad temperature range from tropical, very hot region to arctic weather region. Accordingly, these electrochemical devices for a vehicle are demanded to have no deterioration for the electrochemical properties even when used in a broad temperature range from high temperature to low temperature.
Note that, in the present description, the term of the lithium secondary battery is used as a concept including the so-called lithium ion secondary battery.
A lithium secondary battery mainly consists of a positive electrode and a negative electrode containing materials which can absorb and release lithium, and a nonaqueous electrolytic solution including a lithium salt and a nonaqueous solvent, and as the nonaqueous solvent, a carbonate such as ethylene carbonate (EC) and propylene carbonate (PC) is used.
Further, as the negative electrode, metal lithium, and a metal compound (metal element, oxide, alloy with lithium, etc.) and a carbon material which can absorb and release lithium are known. Particularly, lithium secondary battery produced by using a carbon material, such as coke, artificial graphite, natural graphite and the like which can absorb and release lithium are widely put into practical use.
In a lithium secondary battery produced by using, for example, highly crystallized carbon materials, such as artificial graphites, natural graphites and the like as a negative electrode material, it is known that decomposed products and gases generated from a solvent in a nonaqueous electrolytic solution which is reduced and decomposed on a surface of a negative electrode in charging the battery detract from a desired electrochemical reaction of the battery, so that a cycle property thereof is worsened. Also, when the decomposed products of the nonaqueous solvent are deposited, lithium can not smoothly be absorbed onto and released from a negative electrode, and the electrochemical characteristics thereof are liable to be worsened in a broad temperature range.
Further, in a lithium secondary battery produced by using lithium metal and alloys thereof, metal element, such as tin, silicon and the like and oxides thereof as a negative electrode material, it is known that an initial battery capacity thereof is high but a nonaqueous solvent is acceleratingly reduced and decomposed as compared with a negative electrode of a carbon material since a micronized powdering of the material is promoted during cycles and that battery performances, such as a battery capacity and a cycle property are worsened to a large extent. Also, in a case the micronized powdering of the negative electrode material and the deposition of the decomposed products of the nonaqueous solvent are deposited, lithium can not smoothly be absorbed onto and released from the negative electrode, and the electrochemical characteristics thereof are liable to be worsened in a broad temperature range.
On the other hand, in a lithium secondary battery produced by using, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4 and the like as a positive electrode, it is known that decomposed products and gases generated from a solvent in a nonaqueous electrolytic solution which is partially oxidized and decomposed in a local part on an interface between the positive electrode material and the nonaqueous electrolytic solution in a charging state detract from a desired electrochemical reaction of the battery, so that the electrochemical characteristics thereof are worsened as well in a broad temperature range.
As described above, the decomposed products and gases generated when a nonaqueous electrolytic solution is decomposed on a positive electrode or a negative electrode may interfere with a migration of lithium ions or may swell the battery, and the battery performance is thereby worsened. In spite of the above situations, electronic equipments in which a lithium secondary battery are mounted are advanced more and more in multi-functionalization and tend to be increased in an electric power consumption. As a result thereof, a lithium secondary battery are advanced more and more in an elevation of a capacity, and a nonaqueous electrolytic solution is reduced in a volume thereof occupied in the battery, wherein the electrode is increased in a density, and a useless space volume in the battery is reduced. Accordingly, observed is a situation in which the electrochemical characteristics thereof in a broad temperature range are liable to be worsened by decomposition of only a small amount of the nonaqueous electrolytic solution.
In Patent Document 1, proposed is a nonaqueous electrolytic solution that contains a chain-type saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of a chain-type saturated hydrocarbon compound. For example, it is suggested that an electrolytic solution in which a lithium salt is dissolved in a mixed solvent of 2-methylglutaronitrile:ethylene carbonate:dimethyl carbonate=50:25:25 (volume ratio), has broad potential window.
Further, in Patent Documents 2 and 3, it is disclosed that safety with respect to thermal shock can be improved in a lithium secondary battery using a nonaqueous electrolytic solution containing fluoroethylene carbonate and succinonitrile.